This invention relates generally to cooling a device, for example, a detector, at cryogenic temperatures in a vacuum canister, and more particularly to cooling and protecting a vibrationally sensitive device in a canister at cryogenic temperatures.
A variety of electronic detectors sample electromagnetic waves, for example, x-rays, gamma rays, and infrared radiation, or monitor changes in magnetic fields, for example, superconducting quantum interface devices referred to as SQUIDS. The sensitivities of these detectors can often be greatly enhanced by cooling the detectors to cryogenic temperatures that are generally less than 150 K. Further, the absolute positions of the detector and other equipment in close proximity to the detector must remain fixed in order to minimize detector signal noise errors caused by the slightest motion. Thus, the device which cools the detector must not impose high levels of vibration that would render the detector useless.
In the prior art, a very cold head, a heat absorber, is positioned in an evacuated canister to effect the actual cooling of a detector. Such cooling is frequently accomplished by having a flexible strap, which is also a good thermal conductor, connected between the cold head and the detector. Heat from the detector readily flows to the cold head because of the high conductivity of the strap, and at the same time vibrations generated in the cold head are partially isolated from the detector by the flexibility of the strap connector, e.g., braided copper, between the cold head and the detector.
A desired low temperature for the cold head can be obtained by boil-off of a cryogenic refrigerant, such as liquid nitrogen, in the cold head or in a system closely connected to the cold head. The liquid nitrogen turns to gas as it absorbs heat and the gas is vented. Temperatures in the order of 77 K are provided. However, the boiling action of the liquid nitrogen itself creates vibrations, and thus frequently necessitates an isolation system.
Also, for cryogenic cooling of devices requiring maintenance at such low temperatures refrigerating units of the vapor compression type are frequently utilized. Such systems tend to cause considerable vibration in the cold head to which a device to be cooled, for example, a detector, is directly or indirectly connected. Therefore, these closed cycles have not provided a practical approach where vibration sensitivity is critical.
Historically, braided copper connecting straps have been used to effectively transfer heat from a cooled device where vibration is an important parameter. The straps also provide a flexible linkage between the cold head and the detector that reduces the level of vibration which is imposed on the detector. When the mass of the cooled device is small so that inertial forces are negligible, as is frequently the case with a detector, the vibration level at the detector has a value approximately given by the cold head vibration reduced by the ratio of the stiffness of an interconnecting strap to the stiffness of a support for the detector.
The actual equation is: ##EQU1## where .delta..sub.1 =cold end motion;.delta..sub.2 =detector motion; R1=strap stiffness and R2=support stiffness. When R2&gt;&gt;R1 for good isolation then ##EQU2##
For example, in order to reduce vibration at the detector by a factor of 25 over the vibration level of the cold head, the detector support must be approximately 25 times stiffer than the connecting strap. Such a construction requires a stiff detector support structure, which may lead to unacceptable parasitic heat losses through the structure, and requires a very flexible connecting strap which may lead to low heat transfer efficiency.
Tests have indicated that many vibration-sensitive detectors that operate best at cryogenic temperatures require vibration levels that are less than 0.002 overall grms (where g is the force of gravity) in three axes. In a system wherein the only significant vibration source was the boiling of liquid nitrogen, that is, a system without a vapor compression cycle, the vibration level was found to be marginal at 0.002 grms overall. Thus, to reduce the vibration at the detector, a braided copper strap was used to connect between a cold head of the refrigeration system and the detector, which was supported in a canister. The cold head made contact with the boiling liquid nitrogen.
With this construction, it was possible to reduce the vibration levels at the detector below the 0.002 g level, as required. However, a refrigerant boil-off system presents problems of replacement of exhausted refrigerant supplies, venting of evaporated refrigerant, etc. Thus, it is generally desirable in cryogenic refrigeration systems to rely on a closed cycle vapor compression refrigeration unit when such is available to meet the temperature and heat load requirements.
A closed cycle cooling system, which thermodynamically meets the temperature and load requirements for a detector as was used in the test described above, introduced vibration levels at the cold head which were 10 to 25 times higher than the vibration levels produced by boil-off of liquid nitrogen. To be able to substitute such a vapor compression apparatus, the detector support would require modification to make it 25 times stiffer than the structure required when using the liquid nitrogen boil off system.
Such a detector support construction was impractical to accomplish for use in a typical sensor or detector application. Reducing the stiffness of the interconnecting braided straps was also unacceptable in that thermal problems were induced. A closed cycle refrigeration system for cryogenic temperatures, which was used in this evaluation, is of the type described in U.S. Pat. No. 5,337,572, issued Aug. 15, 1994, and having the same assignee as the present application. This patent is incorporated herein by reference.
What is needed is an improved vibrationally isolated thermal system that may use a closed cycle vapor compression refrigeration unit to cool devices, which operate at cryogenic temperatures and are extremely sensitive to vibrations.